From finding directions to making phone calls, working on laptops to driving hybrid and electric cars, much of life is lived through battery-powered devices. It’s no surprise, then, that developing better batteries is a major thrust of science and engineering research.
Haleh Ardebili, assistant professor of mechanical engineering in the UH Cullen College of Engineering, recently published an article in Nano Letters outlining a technique to improve the performance of lithium ion batteries, which are used in devices such as laptops, cell phones and MP3 players and are even seen as possible power sources for electric cars.
Lithium ion batteries work by passing lithium ions residing in the battery’s anode side through an electrolyte to the cathode side. In doing so, electrons are stripped from lithium in the anode and then travel through an external circuit, providing power. The electrolyte and separator also keep the materials in the anode and cathode sides apart, preventing the two from reacting in possibly dangerous ways while maintaining the overall integrity of the device.
Ardebili’s research involves batteries that utilize solid polymer electrolytes, which offer good thermal and mechanical stability, safety and flexibility but relatively lower ion conductivity compared to liquid electrolytes, meaning lower power output.
Ardebili and collaborators have been able to increase the ion conductivity of these devices by adding to the polymer electrolyte a hybrid filler made of carbon nanotubes insulated by clay platelets.
Microscopy and spectroscopy techniques indicate that the addition of this filler changes how individual polymer chains in the electrolyte interact with each other and with the lithium ions. Typically, some of these polymers come together to form tightly packed crystalline structures. It is difficult for lithium ions to move through such rigid formations. According to Ardebili, the addition of fillers seems to “disrupt the crystallinity by forcing the polymer to wrap around the hybrid nanoparticles. They’re causing irregularity in the packing formation.”
This irregularity allows polymers such as polyethylene oxide to move more freely, making it easier for lithium ions to pass through the electrolyte and thereby increasing power output.
In addition, spectroscopy shows that the nanofillers causes lithium salt in the electrolyte to dissociate at a higher rate. As a result, there are more free lithium ions in the electrolyte, further increasing power output. With an electrolyte that is just 10 percent filled, these two factors combine to increase lithium conductivity by two full orders of magnitude, reports Ardebili.
At the same time, the mechanical strength of these nanofilllers increased the overall resiliency of the polymer electrolyte, with the tensile strength of the electrolyte jumping by 160 percent thanks to the addition. While increased power output is a definite plus, making these polymer electrolytes physically stronger opens the door to some unusual applications, Ardebili said.
“Nanofillers offer optimal properties,” she said. “By enhancing ion conduction and mechanical properties, potential application for these electrolytes include nanosized batteries, batteries that require unconventional shapes or flexible batteries that can be stretched and folded.”